1. Field of the Invention
The present invention relates to calibration of channel gain within multi-channel analog integrated circuits.
2. Description of the Related Art
Recently, considerable interest has developed in analog circuitry that can be shared (multiplexed) across a number of channels. As the number of shared channels increases, e.g., channels shared by a digital signal processor (DSP), the amount of circuit area required by resistive elements becomes critical.
Trim link circuitry embodies resistive elements used to compensate for variances in resistive values (and therefore, gain) typically found throughout replicated portions of semiconductor circuits. To compensate for varying gains, adjustable values of resistor elements are trimmed. Trimming includes blowing fuses within resistive paths to vary the surface area of the resistor formed therewith, thereby up-grading and equalizing device gain.
One example of resistor trimming circuitry is disclosed in U.S. Pat. No. 4,777,471, owned by Precision Micro Devices, Inc., of San Jose, Calif. The trimming circuitry disclosed therein is suitable for adjusting gains within a variety of precision integrated circuits. FIG. 1 shows a portion of trimming circuitry which utilizes avalanche breakdown to fuse trim links. In the figure, a terminal A electrically connects a first end of a resistor R1, a cathode end of zener diode Z.sub.1 and a Pad P1. A second end of resistor R.sub.1 electrically connects to an anode end of diode Z.sub.1, via resistor R.sub.4, and to first ends of resistors R.sub.3 and R.sub.5. Anode end of diode Z.sub.1 electrically connects to an anode end of zener diode Z.sub.2. Cathode end of diode Z.sub.2 electrically connects to a second end of resistor R.sub.5 and a Pad P2, and the second end of resistor R.sub.3 electrically connects to terminal B.
During trimming, a high current pulse is applied from Pad P1 to Pad P2, utilizing the device's inherent avalanche breakdown characteristics and shorting diode Z.sub.1. With Z.sub.1 shorted, R.sub.Ac =R.sub.1 R.sub.4 /(R.sub.1 +R.sub.4). To further trim the resistance, a current pulse is applied from Pad P2 to Pad P1 shorting diode Z.sub.2 and rendering R.sub.AC =1/(1/R.sub.4 +1/R.sub.4 +1/R.sub.5).
While this or similar trimming mechanisms both accurately and efficiently adjust gain, considerable substrate area is consumed to fabricate the required resistors, diodes and pads. As the number of channels increase, the required trimming increases. Increased trimming renders fabrication more complex, both during manufacture and calibration testing. Trimming requirements for multiple channels also increases the number of probe contacts required to make contact with the trim links, the need to verify continuity of the trim links, the frequency of which probe contacts must be cleaned, the large number of relays required for automatic testing, and the time required for testing.